How do you animate material transitions (switching mesh object materials in animation)?
Great opportunity for you to start learning how to use nodes then! 
Because the only way I know for animating a transition between two materials is with nodes.

In the node editor, add the two shaders you want (Shift + A --> Shader), then from the same menu also add a "Mix Shader". Plug in both shader nodes into the mix shader, and animate the Factor to control the transition between the two.

A value of 0 means only the first shader plugged in takes effect, while a value of 1 shows only the 2nd shader. Any value in between is a mixture of the two.

Here's how to do this in Cycles:


In Blender Internal it's a similar thing, only using the Material nodes and the Mix RGB node (preview during animation isn't as good, but when you pause the animation you see the transition):